Moderna has launched a landmark Phase 3 clinical trial of its experimental bird flu vaccine, mRNA-1018, becoming the first mRNA-based pandemic influenza vaccine ever to reach this advanced stage of human testing. The trial, now underway in both the United States and the United Kingdom, marks a critical step in global efforts to prepare for a possible H5N1 bird flu pandemic.
The study will enroll approximately 4,000 healthy adults aged 18 and older across both countries. Of those participants, around 3,000 will be recruited in the UK — with half of the UK participants expected to be over 65 years old — and the vaccine will be administered at 26 sites across England and Scotland. Poultry farmers and people who regularly work with birds are being especially encouraged to sign up, given their elevated risk of exposure to the virus.
What Is mRNA-1018?
Moderna’s mRNA-1018 works similarly to the COVID-19 vaccines that use the same platform. It instructs the body to produce specific proteins linked to the H5N1 virus, effectively teaching the immune system to recognize and fight the pathogen if it ever encounters it. According to Dr. Hiwot Hiruy, Moderna’s senior director of clinical development, early-stage trials showed the vaccine was “generally well tolerated, with most of the side-effects being mild to moderate, and there were no safety concerns.” She noted that a measurable immune response was observed as early as seven days after the first injection and that the response was sustained over time.
Since there are currently very few confirmed cases of bird flu in humans, researchers will rely on immune response data — rather than direct evidence of disease prevention — as an early marker of the vaccine’s effectiveness. The study is sponsored by Moderna and backed by the National Institute for Health and Care Research (NIHR). Participants will receive two doses of the vaccine, spaced three weeks apart, over a seven-month study period.
CEPI Funding Fills a Key Gap
The Phase 3 trial is being funded through an investment of up to $54.3 million from the Coalition for Epidemic Preparedness Innovations (CEPI), a global health partnership dedicated to accelerating vaccine development against epidemic and pandemic threats. CEPI and Moderna confirmed this funding arrangement in December 2025.
The financial support from CEPI came after the U.S. Health Secretary Robert F. Kennedy Jr. canceled more than $700 million in government contracts with Moderna in May 2025, which had been intended to fund the development of a bird flu vaccine for pandemic preparedness. The CEPI investment effectively stepped in to keep the program alive.
Moderna CEO Stéphane Bancel called the start of the Phase 3 trial “a significant milestone,” stating that H5 influenza remains a genuine pandemic threat and that the company’s mRNA platform is critical for rapid, scalable global health responses. CEPI CEO Dr. Richard Hatchett described the trial as “the first-of-its-kind pivotal trial of an mRNA-based pandemic influenza vaccine,” adding that success could “transform our ability to respond swiftly and equitably to one of the world’s most enduring threats.”
Why Scientists Are Watching H5N1 Closely
The A(H5N1) strain of bird flu first emerged in southern China in 1996, with the first recorded human infections occurring the following year. Since 2024, there have been 116 confirmed human cases worldwide, with nearly all of them linked to direct contact with infected animals. The virus has spread beyond birds to infect other species, including mink, marine mammals, and dairy cows in the United States, with several human cases tied to farm workers who had contact with affected cattle.
While experts emphasize that the current risk to the general human population remains low, they warn that the virus is continuing to evolve. Dr. Rebecca Clark, the trial’s national co-ordinating investigator in the UK, said the virus does not yet move easily between humans, but that “we have to treat human-to-human transmission as a real possibility.” She described the trial as a “proactive attempt to shield against that possibility.”
A Faster Tool for Future Pandemics
Dr. Richard Pebody of the UK Health Security Agency (UKHSA) noted that while the timing and cause of the next pandemic remain unknown, a flu pandemic is considered the most likely future threat. He called the mRNA vaccine trial “a key step towards further strengthening our ability to protect people against future influenza pandemics.”
One of the key advantages of mRNA technology is speed. Dr. Hiruy pointed out that mRNA vaccines can be manufactured more rapidly than traditional vaccines and can be adjusted to match new viral strains quickly — a crucial benefit when dealing with a fast-evolving pathogen. The UK government already holds a stockpile of more than five million doses of an H5 influenza vaccine, but that supply relies on conventional vaccine technology. The new mRNA-1018 would serve as an additional layer of protection if approved.
Commitments to Equitable Global Access
Under its agreement with CEPI, Moderna has committed to reserving 20% of its H5 pandemic vaccine manufacturing capacity for low- and middle-income countries at affordable prices — but only if the vaccine is licensed and a flu pandemic is formally declared. If the Phase 3 trial succeeds, regulatory submissions for mRNA-1018 will also be supported by positive data from Moderna’s separate seasonal influenza vaccine candidate, mRNA-1010, which is already under regulatory review in the U.S., the European Union, Canada, and Australia.
